\documentclass{article}
\usepackage[spanish]{babel}

\usepackage{tikz}
\usetikzlibrary{matrix,chains,positioning,decorations.pathreplacing,arrows}

\usepackage[margin=0in]{geometry}

\begin{document}

\begin{tikzpicture}[
init/.style={
  draw,
  circle,
  inner sep=2pt,
  font=\Huge,
  join = by -latex
},
squa/.style={
  draw,
  inner sep=2pt,
  font=\Large,
  join = by -latex
},
start chain=2,node distance=13mm
]
\node[on chain=2] 
  (x2) {...};
\node[on chain=2,join=by o-latex] 
  {...};
\node[on chain=2,init] (sigma) 
  {$\displaystyle\Sigma$};
\node[on chain=2,squa,label=above:{\parbox{2cm}{\centering Funci\'{o}n de \\ propagaci\'{o}n}}]   
  {$f(z)=\frac{1}{1+e^{-z}}$};
\node[on chain=2,label=above:Salida,join=by -latex] 
  {$y$};
\begin{scope}[start chain=1]
\node[on chain=1] at (0,1.5cm) 
  (x1) {$x_1$};
\node[on chain=1,join=by o-latex] 
  (w1) {$w_1$};
\end{scope}
\begin{scope}[start chain=3]
\node[on chain=3] at (0,-1.5cm) 
  (x3) {$x_n$};
\node[on chain=3,label=below:,join=by o-latex] 
  (w3) {$w_n$};
\node[on chain=3] at (3.9,-1.3cm) 
  (salida) {\LARGE $y=f\left( \sum\limits_{i=1}^n {w_i.x_i}\right)$};
\end{scope}

\node[label=above:\parbox{2cm}{\centering Umbral \\ $b$}] at (sigma|-w1) (b) {};

\draw[-latex] (w1) -- (sigma);
\draw[-latex] (w3) -- (sigma);
\draw[o-latex] (b) -- (sigma);

\draw[decorate,decoration={brace,mirror}] (x1.north west) -- node[left=10pt] {Entradas} (x3.south west);
\end{tikzpicture}

\end{document}